<p>My guess would be an intermittent mirror or shutter problem.</p>
<p>You can eliminate metering problems by going outside on a bright sunny day, manually setting the ISO to 100 (or load ISO 100 slide film in the camera), set the aperture to f/16, and meter a gray card in full sunlight. You should get a shutter speed close to 1/100 second.<br /><br />You can eliminate aperture problems by taking identical exposures with different lenses. If the problems happen with only one lens, you found the culprit. If it happens with all the lenses the problem is with your camera. It could be the camera to lens contacts aren't good, but I wouldn't think that would cause your particular problem. It would still be worthwhile to clean the contacts in the camera (pins) and lens contacts.<br>
So that brings us back to shutter or mirror problems and the advice others have given you.</p>
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